Patent GB-512444-A (GB)
Abstract: 512,444. Current and resistance measurements. MERCER, R. (Weston Electrical Instrument Corporation). March 4, 1938, No. 6812. [Class 37] A moving-coil ratiometer having a normally stationary magnetic core disposed between the pole faces of a stationary permanent magnet so as to provide an air gap having an asymmetrical flux distribution has the core mounted so as to be adjustable with respect to the pole faces. The permanent magnet 5 and soft iron pole-pieces 6 are clamped to a base 3 by nuts 7 on studs 4, which also secure the base 3 to the case 1. The moving system comprises two coils 8, 9, Fig. 3, mounted in the same plane and secured to each other and to the bases 10 of the pivots 11 by adhesive material. The C-shaped core 25 between the pole-pieces 6 is so arranged that the air gap in which the coils 8, 9 move varies with their annular displacement, and is adjustable in order to obtain a desired scale. The core 25 is secured to a bracket 26 by a screw 27. The bracket 26 has elongated slots 28, Fig. 7, for receiving bolts or studs 29 that secure the bracket to the pole-pieces 6. The core 25 is thus adjustable from the position shown in Fig. 7, in which the minimum air gap is at the inner edges of the pole faces, to a position in which the maximum air gap is at that regiou. The core 25 is guided to ensure symmetrical variation of the air gaps at opposite sides by a stud 30 engaging the slotted open end of the core and a lug 31 engaging a slot in the core. A screw 32 is provided for clamping the core in its adjusted position. In a modification, Fig. 10, the core 25 is adjusted by means of a threaded stud 34 passing through a bracket 36 secured to the pole pieces. A spring 35 surrounds the stud and bears on the bracket 36 and the core 25. The core is guided by studs 39 passing through holes in the bracket 36. The core may be made of non-uniform crosssection, for example by making the bore eccentric or by inclining the end faces to each other. The permanent magnet core may be of material of high coercive force. For temperature measurements the instrument may be arranged as shown in Fig. 13, in which the resistances of the coils 8, 9 are indicated by series resistors 8<1>, 9<1>. Fixed resistors 41, 42 have a common terminal and their outer terminals are connected to the outer ends of the coils 8, 9. A fixed resistance 43 is connected to the junction of the resistances 41, 42 and a terminal 44. A second terminal 46 is connected through a resistor 47 to the outer terminal of coil 9, the outer terminal of the coil 8 being directly connected to a third terminal 48. A temperature-variable resistance 49 is connected between the terminals 46, 48 so as to form the fourth arm of the bridge of which the other three arms are formed by the resistances 41, 41, 47. A source of potential 45 is connected between the terminals 44, 46. A variable resistance 50 is connected between the junction of the coils 8, 9 and the junction of the resistances 41, 42 for calibrating the instrument readings to conform to a fixed scale of graduations. All the resistances except the thermometer resistance 49 are mounted on the inner face of the base 3 within the instrument casing. For the measurement of temperatures at different points, for example in aircraft engines, a number of such instruments may be connected to a common source of potential, such as a transformer and rectifier unit, one side of each of the thermometer resistances 49 and the corresponding instrument terminals 46 being earthed. Fig. 15 shows an arrangement for indicating liquid levels, in which one pair of bridge arms is formed by fixed resistors 51, 52 and a resistor 53 along which a tap 54 is moved by a float 55.